Usher said he has been preparing to play Leonard -- a fighter with fast hands, smooth feet and a wide smile -- for more than a year and still needs to lose nearly 25 pounds before shooting begins in October.
The movie is based on Duran, a world champion in four weight divisions over a career that spanned five decades.
Duran, whose nickname was "Hands of Stone," rose from the slums of Panama to defeat an unbeaten Leonard in 1980 and claim the WBC welterweight title, only to lose the rematch several months later in the infamous "no mas" fight.
Usher said he was honored to play Leonard.
"You couldn't find a more stylized boxer than Sugar Ray Leonard," Usher said. "He was an incredible motion guy, the way he moved around the ring, and I think my dancing will make it easier for me to pick up his moves. I've been working on familiarizing myself with the ring, sparring and just understanding how to move in the ring."
The cast also includes Robert De Niro as Duran's Hall of Fame trainer, Ray Arcel. Edgar Ramirez, who was recently in "Zero Dark Thirty," will play Duran.
The biopic, written and directed by Jonathan Jakubowicz, is a love story and sports drama that includes the "Brawl in Montreal," the first fight between Duran and Leonard and the "no mas" fight in Orleans.
In the eighth round of that fight, Duran inexplicably quit, saying in Spanish "no mas," which means "no more." They fought one more time, and Leonard won again.
Usher said the rivalry between Leonard and Duran was one of the most exciting and unforgettable in sports.
"It's an incredible story to be told," he said. "The metaphor of determination and struggle is one we can all relate to."
Usher said he has had long conversations with Leonard and will meet him in person in coming weeks.
"He had such incredible discipline, and he was flashy and classy at the same time," said Usher, who's been studying videos from Leonard's boxing days. "I want to be as passionate and dedicated in preparing for this role as he was every time he stepped into the ring."
Usher is an eight-time Grammy-winner with more than 50 million records sold worldwide.
He's a celebrity mentor on the reality TV show "The Voice." He made his Broadway debut in 2006 in "Chicago" as lawyer Billy Flynn and recently appeared in the off-Broadway play "Feurza Bruta."
This will be his first leading role in a feature film.
Usher said he was thrilled to collaborate with De Niro, who will play Arcel, the first trainer to be elected to Hall of Fame.
"Hands of Stone" will be shot entirely in Panama.
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